Problem: Which of these numbers is composite? ${2,\ 7,\ 15,\ 31,\ 43}$
Answer: A composite number is a number that has more than two factors (including 1 and itself). 2, 7, 31, and 43 each have only two factors. The factors of 15 are 1, 3, 5, and 15. Thus, 15 is the composite number.